The invention relates generally to an inkjet printer; and more generally, to a method and system for reducing banding effects in such printers.
In designing a printer, it is important to provide as economically and simply as possible a relatively high output quality at a relatively high speed. The output quality of the printer is a function of its printhead resolution (i.e., the ability to resolve or separate, often visually, two image elements or details). The finer the resolution, the better the print quality.
The resolution of the printhead of an inkjet printer is directly related to its nozzle pitch (i.e., distance between adjacent nozzles on the printhead). This distance is typically designed to be uniform. This generally translates into having a uniform dot pitch (i.e., distance between adjacent dots in a printout). One disadvantage of a uniform dot pitch is the inability to hide printing errors. For example, to print on a print medium, nozzles of the inkjet printer eject tiny droplets of ink, or dots, during each horizontal pass of the printhead over the print medium to form a row of dots. (Each horizontal pass of a printhead over a print medium is called a swath.) After each preceding swath, the print medium is incrementally advanced to allow room for the next row of dots. Through a succession of rows of dots, images or letters are printed on the print medium.
Each dot has a uniform diameter and is placed at a uniform distance from each other on the print medium and each preceding row of dots is placed at the same distance from the succeeding row of dots. Consequently, any two adjacent rows of dots are at the same distance from each other. Hence, there is a uniform band of white space between any two adjacent rows of dots. It should be noted that dot placement in present day inkjet printers is consistent enough that this uniform band of white space is maintained even in more than one pass print mode.
In any case, any small dot placement errors and dot shape variations that impinge on this band of white space create a hue or saturation shift. This hue or saturation shift is often referred to as banding since it exhibits itself as a visible band. This phenomenon is more discernible when printing graphic images.
One way to address banding effects is to eliminate altogether print mechanism errors such as feed errors, nozzle to nozzle dot placement errors, shape variations and the like that foster dot placement errors and dot shape variations. To do so, however, requires the use of very sophisticated programming techniques and precisely engineering mechanical parts. This is an expensive endeavor which is often times reflected in the price of the printer thus built. Therefore, instead of attempting to eliminate these print mechanism errors, a less expensive method is to distribute these errors in such a way as to minimize their conspicuousness (i.e., to hide the errors).
One method that has been used to distribute these errors is to increase the number of swath passes. That is, since sophisticated programming techniques and precisely engineering mechanical parts are not used, line feed errors, nozzle to nozzle dot placement errors, shape variations etc. do occur, albeit very minimally, at each swath pass. The higher the number of passes, therefore, the more prevalent these errors and; consequently, the less uniform the dot pitch (i.e., more and more of the dots appear to impinge on the band of white space). After a certain number of passes (this number is dependent on a particular style of printer), so much of the band of white space is impinged upon, that it ceases to exist. Banding is thus effectively eliminated.
There are a few disadvantages associated with the above-described method. For example, as is obvious, the higher the number of swath passes that are used in printing a page, the slower the printer. Furthermore, the placement of the dots in the band of white space is unpredictable.
Accordingly, it would be beneficial to develop a method that distributes mechanical printing errors in a known and predictable fashion over the band of white space without affecting printer speed and cost.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention is embodied in a printing system for improving the edge quality of ink drops produced by an inkjet printer.
The need in the art is addressed by the present invention. The present invention provides an inkjet printer with the requisite technology to reduce or eliminate altogether banding effects. The inkjet printer includes an input tray for storing print media, an output area for holding printed media and an inkjet pen or printhead for printing information on the print media. In a preferred embodiment, the inkjet pen scans and ejects ink on the print media with a non-uniform dot pitch. This non-uniformity is achieved by adding visual noise in a known and predictable fashion to the uniform dot pitch.